1. Field of the Invention
This invention relates to exposure meters for use with photographic cameras, and, more particularly, to a digital exposure meter in which analog quantities are converted to corresponding digital quantities based on which a proper exposure value is derived by computation.
2. Description of the Prior Art
The prior art provides many methods of deriving exposure values for use in photography, most of which may be grouped into the following three types:
(1) Derivation of an effective exposure aperture for the photographic lens as a result of computation from the object brightness, film sensitivity and shutter speed;
(2) Derivation of an effective exposure time as a result of computation from the object brightness, film sensitivity and aperture value of the photographic lens;
(3) Derivation of an exposure value for a given photographic situation as a result of computation from the object brightness and film sensitivity.
All of these types have respective characteristics and advantages, and have so far been put into practice, but with some drawbacks.
The method of type (1) described above, because of its possibility of preselection of a desired shutter speed, permits the photographer who encounters a given photographic situation associated with a high possibility of introducing vibrations into the hand-held camera, when snap photography is to be made, or when an object which is moving at a high speed is included therein, or when a telephoto objective lens is used with the camera, to take into account the movement of the object and the magnitude of the vibrations. This is done so as to select a relatively faster shutter speed but does not enable him to observe and evaluate in detail the far limit and the near limit of the depth of the total scene prior to making an exposure, because the effective exposure aperture is automatically determined with the preselected shutter speed. Therefore, this first type is not suitable for use in taking a picture of landscape or still life of which the quality is largely dependent upon adjustment of the depth of field, or taking a vignette by making use of the controlled depth of field.
The method of type (2) described above operates in an entirely opposite sense to that of the type (1) method, permitting for the photographer who is about to take a picture of landscape or still life, or to take a vignette, to adjust the size of diaphragm aperture by taking into account the depth of field, but will result in a reduction of the percentage of photographs taken with relatively long shutter speeds which will be found acceptable particularly when the given photographic situation includes an object which is moving at a relatively high speed, or when the camera is hand-held. Therefore, this second type is not suitable for use in taking a picture of an object being moved at a fast speed, or taking picture while the camera is being held by hand.
The method of type (3) described above provides an exposure value which when used in making an actual exposure must be factored into effective exposure time and aperture based on his personal experience and insight.
Accordingly, it is desirable to provide an exposure meter capable of selectively performing either of the methods of types (1) and (2) by the free choice of the photographer as the controls of shutter speed and aperture value have different imaging effects from each other. With this in mind, an analog exposure meter has been proposed which is provided with a function of deriving an effective exposure aperture in automatic response to the sensitivity of the used film, the preselected shutter speed and the level of brightness of an object being photographed, and with a function of deriving an effective exposure time in automatic response to the sensitivity of the used film, the preselected aperture and the object brightness level. This analog exposure meter necessitates an electrical circuit for processing photographic information in the analog form to compute an exposure value, thereby giving a disadvantage of making it practically impossible to manufacture a production run of circuit elements thereof economically while holding all of the circuit parameters to specific and ideal values.
In connection with the method of type (3), there are two metering modes, namely, a spot-metering mode and an overall field of view light metering mode. The spot-metering mode when applied to an exposure determination for that portion of a scene being photographed which constitutes a subject of principal interest provides a correct exposure as the environment of the subject is of no importance. But, when the area of the image within the spot of the subject of principal interest is larger than the area of the spot based on which the exposure determination is made, it is generally impossible to make a correct exposure for the entire area of the subject. The overall field of view light metering mode works in an entirely opposite sense to that of the spot-metering mode. It is known in the art to provide a camera of which light metering aspects may be selectively operated in a spot-metering mode or overall field of view light metering mode. In this case, it is also necessary to process photographic information in the analog form to compute an exposure value. Accordingly, likewise as above, the computing circuit is made complicated in structure, and a requirement for high accuracy of light metering is not satisfied.